The use of thermoplastic (hot melt) media in processes for decorating vitreous, glass-ceramic, and ceramic ware is well established. The principal technique utilized for the application of thermoplastic inks to such substrates has been screening, although other techniques, including transfer printing from an elastomeric surface, such as is disclosed in U.S. Pat. No. 3,688,695, have been described. The major advantage exhibited by thermoplastic inks is their capability to harden quickly upon contact with the surface of ware at room temperature so that, consequently, a second print can be applied over and/or adjacent to a first print without any intermediate drying step, such as is demanded for oil, organic solvent, or water base inks. The compositions comprising these thermoplastic inks have varied widely, but most have displayed wax-like characteristics with low melt viscosities, commonly less than 150 poises at their application temperature, and non-tacky surfaces when cooled substantially below the melting points thereof. The printing pastes or inks commonly consist of a vitrifiable, inorganic material or flux containing a suitable inorganic pigment or coloring material and an organic, flowable vehicle or binder in which the pigment is dispersed. Such pigmented flux materials are well known to the art of decorating glass, glass-ceramic, and ceramic ware, and are supplied by ceramic color manufacturers as complete vitrifiable or ceramic colors which are then dispersed in the vehicle or medium by the user in accordance with his requirements.
When employed in the decorating of glass, glass-ceramic, or ceramic surfaces, the inks are customarily filled with from about 60-90% by weight total of glass fluxes, inorganic pigments, and inorganic opacifiers. The inks so compounded and applied are subsequently fired at high temperatures (over .about.500.degree. C.) to burn out the organic constituents and melt the glass to form indelible markings on the surface of the ware. Consequently, it is obviously essential that the organic components utilized in the ink formulations have the necessary firing characteristics to permit them to be burned off at relatively low temperatures, thereby enabling their removal without deleterious effects upon the visual appearance and intrinsic properties of the fired inorganic decoration. Materials which have commonly been used therefor have included waxes such as fatty alcohols, polyethylene glycol, polyethylene glycol esters, etc., fortified with a cohesive strength imparting resin or viscosity modifier such as ethyl cellulose, hydroxypropyl cellulose, butyl methacrylate, etc.
Inks so formulated are satisfactory for screening processes but are not adequate for numerous other processes, especially those techniques employing transfer (offset) elastomers, such as offset screening or offset stamping from an intaglio plate. A representative process is discussed in U.S. Pat. No. 3,756,165. These offset printing processes were developed for use with conventional oil-based or solvent-based decorating inks. Such inks must dry from a liquid to a tacky, semi-solid state upon the elastomer surface after which they can be transferred as an integral film to a substrate. The thickening effect of the glass fluxes and pigments upon the viscosity of the ink requires the use of relatively high levels of organic solvents which, in turn, must demonstrate relatively high rates of volatilization so as not to limit process speeds and/or saturate the surface of the elastomer. However, the use of solvents having high rates of evaporation causes a viscosity stability problem which is of such magnitude that relatively slow process speeds are frequently resorted to as the lesser evil. This circumstance is exemplified in U.S. Pat. No. 3,756,165.
The advantages inherent in the use of thermoplastic inks for overcoming the viscosity problem are quite obvious, but the thermoplastic formulations typically utilized for screening were unsatisfactory for transfer (offset) printing because of their essential lack of pressure sensitivity at temperatures substantially below their melt points. This situation required the inks to either be heated to temperatures closely approaching their melt points during transfer or be transferred to an adhesive-coated ware surface. This circumstance is described in U.S. Pat. No. 3,688,695.
The art of decorating vitreous, glass-ceramic, and ceramic articles utilizing decalcomania transfers (decals) is also not of recent origin. Thus, decals produced from vitrifiable or ceramic colors have been used for a number of years to decorate glassware, chinaware, pottery, and the like.
The most common type of decalcomania has comprised a paper carrier or backing having the desired design imprinted thereupon in one or more vitrifiable colors. Water release decals have been extensively employed which are either of the slide-off type or the varnish-applied, duplex paper type. The slide-off decal utilizes a backing paper having a thin layer of a water soluble gum with the vitreous design imprinted face up upon this gummed surface. A clear supporting film, commonly of nitrocellulose, is applied over the design layer. The duplex paper type consists of a layer of thin tissue paper releasably mounted on a heavier paper sheet. The tissue paper carries a water soluble gum coating and the vitreous design layer is imprinted face down upon the gum coating. As the name suggests, it is necessary to soak these decalcomania with water to effect separation of the backing from the design. After the design has been temporarily attached to the ceramic substrate, the article is dried thoroughly, following which the article is fired at an elevated temperature to fuse the design into the surface thereof.
A comparatively recent development in the field of decalcomania has been the use of a heat release-type, vitreous or ceramic decal. These products consist of a heat releasable backing at the face or front side of the vitreous design layer and an outermost thermoplastic or heat activatable adhesive surface at the opposite or rear side of the vitreous design. When the outermost thermoplastic or heat activatable surface of the decal is pressed against the surface of a preheated vitreous or ceramic article, the heat of the article softens the adhesive surfaces of the decal to a sufficient extent such that the design is temporarily adhered to the article being decorated. Concurrently, the heat from the article softens or melts the heat release layer of the backing, thereby causing the backing to release from the design layer. Both actions are accomplished in essentially a single operation in which the decal is urged against the preheated article. Frequently, the released backing sheet will then be fully removed from the article utilizing a jet of air or the like. The ware with the temporarily adhered vitreous design is thereafter fired in the normal manner to cause the design to become an integral part of the surface of the ware.
Various modifications and improvements have been made to the basic structure or makeup of heat release decals. For example, the design layer may be either a single layer or, more frequently, a composite layer comprising several different ceramic color compositions arranged to provide the desired ornamentation or textual matter. Where desired to impart strength and integrity to the design layer or to supply an outermost surface that can be more readily rendered tacky or adhesive, another layer may be deposited over the vitreous design. This additional layer may be of a resinous material exhibiting thermoplastic properties or some special heat-activatable, thermoplastic adhesive material.
The term "thermoplastic" as employed in the context of this adhesive layer is to be distinguished from the term as applied above in connection with thermoplastic inks. In both instances, the materials reversibly soften with heat. Hence, the term "thermoplastic." However, when used in the context of inks, the term also implies melt processibility or application; whereas the material utilized for the decal adhesive layer is merely applied from solution over the vitreous design. Accordingly, to avoid ambiguity in the term "thermoplastic," melt processible inks are sometimes referred to as "hot melt inks" or simply "hot color;" whereas the solution-applied adhesive overlay is frequently referred to as a "lacquer."
The releasable backing of the conventional, heat release vitreous decalcomania generally comprises a paper sheet having a barrier layer or coating which renders one side of the paper less porous and retards penetration by molten wax or wax-like materials. Superjacent to the barrier layer is a coating or film of a wax-like, heat release material. Next above the heat release material is a clear film which serves as an imprint-receiving support for the subsequently applied vitreous design. Finally, if desired, the above-described strengthening and/or adhesive layer can be applied over the design.
A third type of vitreous or ceramic decalcomania is known in which the design layer releases from the paper backing solely upon the application of pressure. These decals have been defined in such terms as pressure release, cold release, and dry release. This type of decal shall be referred to herein as pressure release decals.
The structure of these decals can be similar to a heat release decal but wherein the wax release layer is replaced with a silicone release layer. Furthermore, the adhesive layer overlay must also demonstrate room temperature pressure sensitivity thereby obviating the need for heating to cause adherence of the decal to the surface of the ware. Because of the preferential adherence of the adhesive layer to a vitreous or ceramic surface rather than the silicone release layer, release of the decal is secured by merely pressing the decal against the ware to be decorated.